As electric linear motion actuators of this type including an electric motor as a driving source, electric linear motion actuators as disclosed in the below-identified Patent documents 1 and 2 are known.
In the electric linear motion actuators disclosed in Patent documents 1 and 2, planetary rollers are incorporated between a rotary shaft configured to be rotated by an electric motor and an axially movably supported outer ring member, when the rotary shafts rotates, the respective planetary rollers rotate about their axes while revolving around the rotary shaft due to the frictional contact of the planetary rollers with the rotary shaft, and a helical rib formed on the inner diameter surface of the outer ring member engages in a helical groove or circumferential grooves formed in the outer diameter surface of each planetary roller, thereby moving the outer ring member in the axial direction.
The electric brake systems in which the electric linear motion actuators disclosed in Patent documents 1 and 2 are incorporated have only a service brake function for controlling a braking force according to how much the driver of the vehicle operates the brake pedal. As a result thereof, since while the vehicle is parked, a braking force needs to be maintained by keeping the electric motor energized, there is the disadvantage that the power consumption of the electric motor is large.
In order to overcome this disadvantage, the applicant of the present invention has already proposed in the below-identified Patent document 3 an electric linear motion actuator capable of maintaining a braking force even while an electric motor is de-energized.
The electric linear motion actuator disclosed in Patent document 3 includes a reduction gear mechanism in which the rotation of a rotor shaft of an electric motor is reduced in speed and transmitted to a rotary shaft. The reduction gear mechanism is constituted by a plurality of gears, and one of the gears is formed in its side surface with a plurality of circumferentially spaced apart engagement portions. By activating a linear solenoid, a locking pin movable toward and away from one of the engagement portions advances so as to engage in one of the engagement portions and thus to lock the one of the gears formed with the engagement portions, thereby making it possible to maintain a braking force even while the electric motor is de-energized.
By using in an electric brake system such an electric linear motion actuator as described above, while the vehicle is parked, it is possible to lock brake pads with the brake pads pressed against a disk rotor by a predetermined pressing force, and to make the electric linear motion actuator small-sized.